Revert :visibility property to definition in Observable namespace

Reported in CASE-Mapping-Python PR 38.

No effects were observed on Make-managed files.

References:
* casework/CASE-Mapping-Python#38

Reported-by: Fabrizio Turchi <fabrizio.turchi@cnr.it>
Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

Author: ajnelson-nist

examples/illustrations/message/message.json

@@ -165,7 +165,7 @@
                     "@type": "uco-observable:MessageThreadFacet",
                     "identifier": "billy~sarah@whatsapp.gs.net",
                     "uco-observable:displayName": "Best Friend Chat!!",
-                    "drafting:visibility": "PRIVATE",
+                    "uco-observable:visibility": false,
                     "uco-observable:messageThread": {
                         "@id": "kb:thread-c5b68f3b-a9a1-4595-9d54-b3fd3605871e",
                         "@type": "uco-types:Thread",
@@ -246,7 +246,7 @@
                     "@type": "uco-observable:MessageThreadFacet",
                     "identifier": "twitter_public",
                     "uco-observable:displayName": "Argle-bargle",
-                    "drafting:visibility": "PUBLIC",
+                    "uco-observable:visibility": true,
                     "uco-observable:messageThread": {
                         "@id": "kb:thread-f8ebd1db-ce49-495e-850a-7b44187cb128",
                         "@type": "uco-types:Thread",